You had to ask – well, here goes.
The restrictions are currently as follows, but note that they do look a bit contrived, resulting from the fact that they have grown over time (from single site to cross-site, from "there are no subsites" to "facebook.SO", etc.), so it's not unlikely that we'll revisit them.
- Links to questions on the same site are unlimited.
- Links to other sites are limited to
- a) 5 linked-to sites and
- b) N questions per target site, where N is 40 on meta sites and 10 on non-metas (this is about the site that the post containing the link is posted on, not the site that is being linked to)
- While same-site links always work, which other sites work depends on where you're posting:
- a) On all subsites (currently none), links to the master site work, and vice versa.
- b) On all meta sites, links to the corresponding main site work, and vice versa.
- c) On Meta Stack Exchange, all sites work.
- d) On all meta sites, links to Meta Stack Exchange work.
- e) Anything else does not work.
- For the purpose of 2 (in contrast to 1 and 3a), facebook.SO and SO were considered separate sites.
The reason that removing the
facebook. part from links caused the auto-title to suddenly work, as you observed, is 2b in combination with 4: forty links to fb.so work; the 41st doesn't.
Point 4 is also the rule that makes the least sense, so don't expect that to stick around forever.
This will also work identical in the preview from the next build on, with the exception of 2a. That one is only applicable to Meta SO anyway (because of 3), it's non-trivial to add to the preview because of slight differences in the implementation, and it's a pathology to begin with.
To answer your other question in addition to what Arjan said: on the server-side, same-site links are retrieved from the database, cross-site links use an HTTP API. The latter used to happen via cross-DB queries, but this was dropped at some point, because having multiple database connections lying around is just crying for issues.